Semiconductor laser diodes of the type mentioned above have, for example, become important components in the technology of optical communication, particularly because such lasers can be used for amplifying optical signals immediately by optical means. This allows to design all-optical fiber communication systems, avoiding any complicated conversion of the signals to be transmitted, which improves speed as well as reliability within such systems.
In one kind of optical fiber communication systems, the laser diodes are used for pumping Erbium-doped fiber amplifiers, so-called EDFAs, which have been described in various patents and publications known to the person skilled in the art. An example of some technical significance are ridge-waveguide laser diodes with a power output of 150 mW or more, whose wavelengths match the Erbium absorption lines and thus achieve a low-noise amplification. Several laser diodes have been found to serve this purpose well and are used today in significant numbers. However, the invention is in no way limited to such laser diodes, but applicable to any ridge-waveguide laser diode.
Generally, laser diode pump sources used in fiber amplifier applications are working in single transverse mode for efficient coupling into single-mode fibers and are mostly multiple longitudinal mode lasers, i.e. Fabry-Perot (or FP) lasers. Three main types are typically being used for Erbium amplifiers, corresponding to the absorption wavelengths of Erbium: InGaAsP at 1480 nm; strained quantum-well InGaAs/AlGaAs laser diodes at 980 nm; and AlGaAs laser diodes at 820 nm.
One of the major problems of semiconductor laser diodes of the types mentioned above is the degradation in the end section area, in particular at the front facet of the laser. This degradation is believed to be caused by uncontrolled temperature increase at the mirror facet regions, especially at high power outputs, which temperature increase in turn is probably caused by unwanted carrier recombination in these regions and heating due to free carrier injection.
Consequently, ways have been sought to prevent this carrier recombination in the laser diode's facet regions. One attempt is described in Itaya et al U.S. Pat. No. 5,343,468. It discloses a compound semiconductor laser diode with a current blocking region formed in one facet portion of the laser structure. Though this design may be advantageous for the kind of laser diodes addressed by Itaya, namely regrown/buried double heterostructure laser diodes, it is not manufacturable for ridge waveguide laser diodes of the kind addressed by the present invention. A further problem occurs when manufacturing AlGaAs laser diodes with a two step epitaxial process. Here, the quick oxidation of Al seriously interferes with the Itaya process and thus makes it rather unsuitable for industrial application.
Yu et al. U.S. Pat. No. 6,373,875 discloses a plurality of current-blocking layers, one each over each of the end sections of the laser's ridge waveguide and two separate blocking layers fully covering the remaining body right and left of the ridge waveguide. This structure thus has several layers which are laterally not contiguous and the interruption just at the edge of the waveguide may lead to undesired effects.
Sagawa et al. U.S. Pat. No. 5,844,931 discloses a “windowed” current-blocking layer covering the ridge and the whole body with a longitudinal opening, i.e. a window, over the center part of the ridge. Apart from the fact that some of the current blocking layers in this USP are actually conductive, not isolation layers (as in the present invention), it discloses one single layer fully covering the laser body, with just a window over part of the ridge. Thus, the blocking layer is longitudinally not limited to the end section(s) of the laser. The manufacturing of such a windowed blocking layer process requires very careful alignment, especially of the window, to obtain the desired results and thus appears rather complex. In contrast to that, providing separate isolation layers, one layer over one or both end sections and separate lateral layers on both sides of the ridge waveguide, as the present invention does, enables a more easily manufacturable design, with the lateral isolation layers preferably being formed by a self-aligned process.
Thus, it is the main object of the invention to devise a simple and reliable design for a high power ridge waveguide laser diode which avoids the abovementioned end section degradation to provide a stable light output power under all operating conditions. Another object is to provide an economical manufacturing method, allowing reliable mass production of such laser diodes.
A further aspect is the provision of separate isolation layers, one layer over one or both end sections and separate lateral layers on both sides of the ridge waveguide. The lateral isolation layers extend longitudinally on the semiconductor body along the ridge waveguide, substantially abutting the latter.
A still further object is to avoid adding to the complexity of the laser diode structure and to keep the number of additional structural components of the laser diode at a minimum.